Effect of chemical structure on the hydrolytic stability of lubricating base oils

Base oils differing in chemical structure, representatives of hydrocarbon fluids, ester fluids, and water-insoluble polyalkylene glycols (PAGs), were tested (ASTM D2619) for comparison of their hydrolytic stability, which was found to depend on both their chemical structure and purity. The variations observed after hydrolytic degradation in oil acid numbers, water layer acidity, and copper (Cu) strip weight are indicative of excellent hydrolytic stability of hydrocarbon oils, very good hydrolytic stability of PAG oils, and good hydrolytic stability of polyolester oils (characterized by high purity and an acid number lower than 0.1 mg KOH/g). The positive effect of steric hindrance around the ester bonds on the hydrolytic stability of polyolesters was substantiated by the results obtained for the oils of a pentaerythrite tetracaproate and pentaerythrite tetra(sec-caproate) structure.